The subject invention generally relates to a composition and method for the in situ removal of scale from a substrate. More specifically, the subject invention relates to a composition and method for the in situ removal of silicate-containing scale from interior surfaces of a boiler.
Compositions and methods for removing scale are known in the art. It is understood that steam is produced for various residential, commercial, and industrial purposes. For instance, steam is produced in a heat exchange apparatus, such as a steam boiler, to provide heat for residential, commercial, and industrial buildings. Steam is also produced to generate power for various residential, commercial, and industrial applications. In the production of steam, large quantities of water are heated to temperatures that exceed the boiling point of water. As such, minerals and salts, which are dissolved in the water, are deposited on interior surfaces of the steam boiler. These deposits are known in the art as scale.
Scale is undesirable as it decreases the thermal efficiency of the steam boiler. Scale may also possess corrosive properties that can degrade the interior surfaces of the steam boiler and shorten the useful operating life of the steam boiler. It is generally understood that there are three classifications of scale. First, there is scale, typically an iron oxide, that is generated as the steam boiler corrodes. Second, there is scale that is a phosphate, carbonate, or sulfate of either calcium or magnesium. This second classification of scale is most common. Third, there is scale that is a silicate-sulfate complex of calcium, magnesium, aluminum, and other various metals. This third classification of scale is typically the most difficult scale to remove from the interior surfaces of the steam boiler.
The conventional compositions and methods of the prior art are primarily directed at removal of the first two classifications of scale. For example, conventional compositions include acids targeted to remove phosphate, carbonate, and sulfate scale of calcium and/or magnesium. It is known throughout the art that these acid compositions are ineffective in removing the third classification of scale from the steam boilers. More specifically, these acid compositions are ineffective in removing the silicate-sulfate complexes of calcium, magnesium, aluminum, and other metals. Other conventional compositions rely exclusively on chelants, such as ethylenediaminetetraacetic acid (EDTA) and nitrilotriacetic acid (NTA), to remove the phosphate, carbonate, and sulfate-based scale. As with the acid compositions, these chelant only-based compositions are ineffective in removing the third classification of scale and even require several days to remove the other classifications of scale. Other conventional compositions rely on citric acid for the removal of scale. More specifically, these conventional compositions rely on citric acid in an overall acidic environment, i.e., at pHs less than 7, to remove only the first classification of scale, the iron oxide scale.
Due to the overall ineffectiveness of the above compositions in removing the silicate-sulfate complexes of calcium, magnesium, aluminum, and other metals, the many industries that utilize steam for heat, power, and other reasons, have had to resort to impractical, time-consuming, and expensive methods to prevent the deposition of this third classification of scale. These industries have even had to resort to physical removal of this scale. For example, some in industries must continuously treat their water to remove the minerals and salts that form the scale from the water, or to prevent the deposition of the scale. Other industries take the steam boiler out of service for certain periods of time and employ personnel to physically remove the scale while the steam boiler is out of service.
In sum, the prior art compositions described above are characterized by one or more inadequacy. Due to the inadequacies identified in the prior art, it is desirable to provide a novel composition and method that use simple ingredients, such as citric acid in an overall basic environment established by an alkali metal hydroxides, to effectively remove silicate-containing scale from the interior surfaces of a boiler.
A composition and method for the in situ removal of scale from a substrate are disclosed. Specifically, the composition and method of the subject invention are effective in removing silicate-containing scale, formed as silicate-sulfate complexes of calcium, magnesium, aluminum, and other metals, from the interior surfaces of boilers. The substrate is exposed to the composition to remove the scale. The composition includes a chelating agent (A) and a basic agent (B). The chelating agent (A) has at least two carboxylic acid functional groups and is preferably citric acid. The basic agent (B) is an alkali metal hydroxide. More specifically, the basic agent (B) is preferably potassium hydroxide, sodium hydroxide, or a combination of the two. The chelating agent (A) is present in the composition in an overall basic environment having a pH of from 7 to 14 established by the amount of the alkali metal hydroxide.
The method of the subject invention circulates the composition to contact the substrate. The method also includes other steps, such as heating the composition, filtering the composition, and, if necessary, re-circulating the composition, to ensure sufficient removal of the silicate-containing scale from the substrate. As the composition circulates to contact the substrate, the chelating agent (A) interacts with and extracts atoms of calcium, magnesium, aluminum, and other metals from the silicate-containing scale, and the overall basic environment in the composition enables these metal atoms to precipitate out of the composition as solid particulates.
Accordingly, using simple chemical ingredients, the subject invention provides a novel composition and method including a chelating agent (A) and a basic agent (B) that establishes an overall basic environment for the chelating agent (A) to effectively remove silicate-containing scale from the interior surfaces of a boiler.
The composition and method of the subject invention remove scale from a substrate. More specifically, the composition and method provide for the in situ removal of scale, preferably silicate-containing scale, that is deposited on interior surfaces of various heat exchange apparatus, such as a steam boiler, during normal operation of the apparatus. To remove scale that is deposited on the interior surfaces, the substrate is exposed, in some manner, to the composition. Preferably, the composition is circulated to contact the interior surfaces. Of course, it is to be understood that the composition and method of the subject invention can be applied to any vessel in which an aqueous liquid is handled without varying the scope of the subject invention. The silicate-containing scale that is effectively removed by the composition and method disclosed herein is more completely described as a silicate-sulfate complex of calcium, magnesium, aluminum, and other various metals. An example of such a silicate-sulfate complex is calcium silicate, CaSiO3.
The composition includes a chelating agent (A) and a basic agent (B). The chelating agent (A) has at least two carboxylic acid functional groups, and the basic agent (B) establishes a pH of from 7 to 14 in the composition and is selected from the group consisting of alkali metal hydroxides. The chelating agent (A), the basic agent (B), and the significance of the basic pH are described in detail below.
The chelating agent (A) is selected from the group consisting of compounds having at least two carboxylic acid functional groups (A)(i), compounds having at least two functional groups, other than carboxylic acid functional groups, that are convertible into carboxylic acid functional groups (A)(ii), and combinations thereof. It is to be understood that, without varying the scope of the subject invention, the chelating agent (A) may also be organically or inorganically-substituted. As an example, the chelating agent (A) can include halogen substituents.
The compounds having at least two carboxylic acid functional groups (A)(i) are selected from the group consisting of dicarboxylic acids, polycarboxylic acids, and combinations thereof. Various di and polycarboxylic acids include oxalic acid, adipic acid, malonic acid, succinic acid, glutaric acid, 1,2-benzenedicarboxylic acid (phthalic acid), 1,3-benzenedicarboxylic acid, 1,4-benzenedicarboxylic acid, heptanedioic acid, citric acid, and combinations thereof. In the preferred embodiment of the subject invention, the chelating agent (A) is citric acid. Other equivalent chelating agents (A) having at least two carboxylic acid functional groups include, but are not limited to, diglycolic acid, itaconic acid, malic acid, fumaric acid, glutamic acid, tartaric acid, ethylenediaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA), ethyleneglycol-bis(beta-amino-ethyl ether)-N,N-tetraacetic acid, and combinations thereof.
The compounds having at least two functional groups, other than carboxylic acid functional groups, that are convertible into carboxylic acid functional groups (A)(ii) are selected from the group of dicarboxylic acid derivatives convertible into dicarboxylic acids by hydrolysis, polycarboxylic acid derivatives convertible into polycarboxylic acids by hydrolysis, and combinations thereof. More specifically, the dicarboxylic acid derivatives convertible into dicarboxylic acids by hydrolysis are selected from the group consisting of diacyl halides, cyclic carboxylic acid anhydrides, esters of dicarboxylic acids, dicarboxamides, and combinations thereof. As an example, phthalic anhydride, a cyclic carboxylic acid anhydride and the corresponding acid anhydride of phthalic acid, reacts with water to produce phthalic acid. Therefore, phthalic anhydride is a dicarboxylic acid derivative that is convertible into a dicarboxylic acid by hydrolysis.
The chelating agent (A) is present in the composition in an amount from 5 to 25, preferably from 10 to 20, parts by weight based on 100 parts by weight of total composition. Although not required, the chelating agent (A) is preferably in solution with water when added to the composition. A suitable solution of the chelating agent (A) in water is a 10 to 50 percent solution with water, by weight of the chelating agent (A). The most preferred solution of the chelating agent (A) in water is a 15 to 30 percent solution of the chelating agent (A) with water, by weight of the chelating agent (A).
The chelating agent (A) of the subject invention is at least a bidentate ligand. In other words, the chelating agent (A) is either a bidentate ligand or a polydentate ligand. For example, oxalic acid is a common bidentate ligand that is capable of bonding to a metal atom through two of its four oxygen atoms. Citric acid is a common polydentate ligand that is capable of bonding to a metal atom through more than two oxygen atoms. It is to be understood that the metal atom acted on by the bidentate or the polydentate ligand is either the calcium, magnesium, aluminum, or the other various metals contained within the silicate-containing scale that is removed by the subject composition.
As stated above, the basic agent (B) of the subject invention is an alkali metal (Group IA of the Periodic Table of Elements) hydroxide. That is, the basic agent (B) is a hydroxide of Li, Na, K, Rb, Cs, or Fr. In preferred embodiments of the subject invention, the basic agent (B) is selected from the group consisting of NaOH, KOH, and combinations thereof. In the presence of water, the basic agent (B) ionizes to produce hydroxide ions (OHxe2x88x92) which establish an overall basic environment in the composition. More specifically, the basic agent (B) establishes a pH of from 7 to 14 in the composition. Preferably, the basic agent (B) establishes a pH of from 10 to 14, more preferably from 12 to 14. The basic environment in the composition encourages precipitation of the metals contained within the silicate-containing scale as described below.
The basic agent (B) is present in an amount from 5 to 35, preferably from 10 to 25, parts by weight based on 100 parts by weight of the total composition. As with the chelating agent (A), the basic agent (B) is preferably in solution with water when added to the composition. A suitable solution of the basic agent (B) in water is a 25 to 75 percent solution with water, by weight of the basic agent (B). The most preferred solution of the basic agent (B) in water is a 40 to 60 percent solution of the basic agent (B) with water, by weight of the basic agent (B).
To achieve the basic environment in the composition, the subject invention includes a molar excess of the basic agent (B) relative to the chelating agent (A). This molar excess is from 0.025 to 0.075, preferably from 0.055 to 0.065, excess moles of the basic agent (B). In terms of the aqueous solutions of the chelating agent (A) and of the basic agent (B) set forth above, the volume ratio of a 20 to 25 weight percent aqueous solution of the chelating agent (A) to a 45 to 55 weight percent aqueous solution of the basic agent (B) in the composition is from 1:4 to 4:1. In the most preferred embodiment of the subject invention, the chelating agent (A) is in a 22.5 weight percent aqueous solution, the basic agent (B) is in a 50 weight percent aqueous solution, and the volume ratio of the aqueous solution of the chelating agent (A) to the aqueous solution of the basic agent (B) is 4:1. The volume ratio of the aqueous solutions of the chelating agent (A) and of the basic agent (B) establishes a composition having a boiling temperature of from 100 to 120, preferably from 105 to 115xc2x0 C.
Due to the aqueous solutions of the chelating agent (A) and the basic agent (B), the preferred composition inherently includes water. Based on 100 parts by weight of the total composition, water is present in an amount from 50 to 75 parts by weight. In the preferred embodiment, the water is present is an amount from 55 to 70 parts by weight, and in the most preferred embodiment, the water is present in an amount from 61 to 69 parts by weight.
The composition optionally includes a monodentate ligand bonding to the metal atoms through one of its atoms. If included, the preferred monodentate ligands are selected from the group consisting of carboxylic acids having from 1 to 20 carbon atoms. Examples of suitable carboxylic acids include formic acid, acetic acid, propanoic acid, butanoic acid, pentanoic acid, benzoic acid, and equivalents thereof. One example of other suitable monodentate ligands not having carboxylic acid functionality that may be included in the composition is ammonia.
The composition of the subject invention may further include at least one additive selected from the group consisting of pH indicating dyes, corrosion inhibitors, polymeric dispersants, and combinations thereof. For example, the composition may include phenolphthalein to monitor the basicity of the composition as the substrate is exposed to the composition. Phenolphthalein is generally effective between 10.0 and 8.2 pH. Examples of other suitable pH indicating dyes include, but are not limited to, tropeolin O which is generally effective between 13.0 and 11.0 pH, alizarin yellow R which is generally effective between 12.0 and 10.2 pH, thymolphthalein which is generally effective between 10.6 and 9.4 pH, and nitramine which is generally effective between 13.0 and 10.8 pH. Examples of suitable dispersants include, but are not limited to, Dispersol 1324 and Dispersol 1364 commercially available from Lombardi Water Management of Ohio. Examples of other suitable dispersants include, but are not limited to, Nalco 7362 and Nalco Transport-Plus 7104T commercially available from Nalco Chemical Company of Illinois.
As described above, the basic agent (B) produces and contributes hydroxide ions in the composition. The hydroxide ions chemically attack the silicon atoms in the silicate-containing scale to disrupt the Sixe2x80x94O bonds. More specifically, the hydroxide ions chemically attack the silicon atoms in the silicate-sulfate complexes of the calcium, magnesium, aluminum, and other various metals to disrupt the Sixe2x80x94O bonds. As a result, the metal atoms of the calcium, magnesium, aluminum, and other various metals that are complexed with the Sixe2x80x94O bonds are exposed. Next, the chelating agent (A), in the preferred embodiment citric acid, interacts with and is able to extract the exposed metal atoms from the scale. That is, the chelating agent (A), which is either a bi or polydentate ligand, bonds to the metal atoms. Finally, achieving the overall basic environment in the composition, via the concentration of the hydroxide ions from the basic agent (B), is desirable as basic pHs enable the metal atoms to precipitate out of the composition as solid particulates of metal hydroxides. In fact, if the hydroxide ion concentration is too low, a pH of less than 7, then the effectiveness of the composition of the subject invention is diminished and it is more difficult for the metal atoms to be precipitated out of the composition. After precipitation, the solid particulates of the metal hydroxides, calcium hydroxide, magnesium hydroxide, aluminum hydroxide, and other metal hydroxides, can then be filtered from the composition. Filtering of the composition is described below.
The method disclosed in the subject invention uses the composition set forth above to remove the scale from the substrate. This method is most effective in removing silicate-containing scale previously deposited on the interior surfaces of the boiler. In this method, a first volume of the composition is circulated to contact the substrate. In terms of the steam boiler, the first volume of the composition is first charged into the boiler, and then the first volume is circulated throughout the boiler to contact the various internal surfaces of the boiler. The composition is preferably heated to from 50 to 110xc2x0 C. to increase effectiveness in removing the scale. More preferably, the composition is heated to from 90 to 105xc2x0 C. It is also preferred that the composition is heated during circulation throughout the boiler. However, the composition may alternatively be heated by external sources prior to circulation throughout the boiler.
The composition is circulated for from 1 to 48, preferably from 20 to 30, hours. After the appropriated circulation time, the substrate is inspected to determine if the scale has been removed. If, after inspection, it is determined that there is still additional scale deposited on the substrate that requires removal, then the first volume of the composition can be re-circulated to remove this additional scale from the substrate. If necessary, this re-circulation step is preferably conducted for from 5 to 20, more preferably from 9 to 15, hours, to remove the additional scale. Next, the first volume of the composition is filtered to eliminate the solid particulates of the metal hydroxides. After the first volume has been filtered, the first volume is preferably supplemented with additional basic agent (B).
Alternatively, if it is determined that there is still additional scale deposited on the substrate, then a xe2x80x98freshxe2x80x99 or second volume of the composition may be charged into the boiler and then re-circulated to remove this additional scale. As with the first volume of the composition, the second volume of the composition may be filtered to remove the solid particulates of the metal hydroxides if subsequent re-circulation is necessary.
After circulation of the composition and after an adequate inspection has determined that the scale has been removed, then certain steps are take to prepare the substrate for a return-to-service, i.e., normal operation. The substrate is rinsed with water. Next, the substrate is also passivated as is known in the art to further prepare the substrate for the return-to-service. Passivation of the substrate prevents subsequent degradation of the substrate by corrosion. A passivating solution, comprising hydrochloric acid and sodium nitrate, is circulated to contact the substrate. More specifically, the passivating solution comprises 1 weight percent HCl in water and 1 weight percent sodium nitrite, NaNO2, in water and is preferably circulated for from 1 to 8 hours. In the preferred embodiment where the substrate that is treated is the steam boiler, the passivating solution is circulated throughout the boiler to contact the various internal surfaces of the boiler.